1. Field of the Invention
The present invention relates to a code division multiple access (CDMA) communication system, and in particular, to a handoff control method for a CDMA mobile communication system.
2. Background of the Related Art
Generally, handoff serves to continually maintain a call between a mobile station and a base station when the mobile station moves from a current base station area to an adjacent base station area during the call. The handoff in a code division multiple access (CDMA) mobile communication system can be classified into a softer handoff, a soft handoff and a hard handoff.
The softer handoff is generated during movement between sectors of a base station. The sectors within the base station have the same frequencies and the same frame offset values. For the softer handoff process, a new speech path for an object sector are set while in a current sector an existing speech path between the mobile station and the base station is maintained. Thus, a voice packet is transmitted to a plurality of speech paths during the softer handoff. After the mobile station completely moves to the object sector, if the maintenance of the existing speech path is not required, the resource (e.g., the existing speech path) is retrieved.
The soft handoff is generated during movement between adjacent base stations (i.e., base station cells) where the same frequencies and the same frame offset values are used within each of the base stations. For a soft handoff process, a new speech path for an object base station cell is set while an existing speech path between a mobile station and the current base station is maintained for a predetermined time. Thus, a voice packet is transmitted to a plurality of speech paths. After completely moving to the object base station cell, if the maintenance of the existing speech path is not required, the resource (e.g., existing speech path) is retrieved.
The hard handoff is generated in the case where a frame offset value or a frequency is altered during movement between adjacent base stations and when a CDMA mobile communication system is moved to an analog system. For a hard handoff process, an existing speech path can not be maintained in contrast to the above discussed softer handoff and soft handoff processes. Accordingly, only the new speech path is used to transmit a voice packet.
In the CDMA mobile communication system, a handoff control signal procedure between the mobile station and the base station including a control station is processed using three messages in a mobile station assisting handoff system. First, the mobile station monitors the same frequency pilot signal strength of an adjacent cell to transmit a pilot strength measurement message (PSMM). Second, after analyzing the PSMM and determining whether another speech path is constituted or any one among pre-established speech paths is retrieved, the base station and the control station transmit an extended handoff direction message (EHDM) to the mobile station to instruct the handoff. Third, the mobile station receiving the EHDM takes the action corresponding to the instruction of the base station and the control station and as a result transmits a handoff completion message (HCM). For example, the mobile station can establish or retrieve a new speech path. During the procedure of the handoff control signal, the adjacent base station, which receives the PSMM from the mobile station upon starting the handoff, should have the same frequency as during the initial call.
FIG. 1 is a diagram illustrating a multiple frequency environment of an exemplary CDMA system. The CDMA system includes an A terminal base station 104 that exists in a boundary area of the multiple frequency environment using Fh group frequencies and Fn group frequencies in a subscriber overcrowded area, an adjacent B base station 105 and a C base station 106, which uses a small Fh group frequencies like the terminal base station but in a subscriber non-overcrowded area. A Base Station Controller (BSC) 102 controls the A terminal base station 104 and the adjacent B base station 105. A Base Station Conttroller 103 controls the C base station 106. An Mobile Switching Center (MSC) 100 exchanges communications between the Base Station Controllers 102 and 103 and a location register 101 registers positions of mobile stations such as mobile stations 107 and 108. The Mobile Switching Center 100 can further be coupled to control communications with a public switched telephone network (not shown) or the like.
When the base station uses a different frequency from the adjacent base station in the multiple frequency environment of a CDMA system, the mobile station can not transmit the PSMM for the adjacent base station so the handoff can not be achieved. In other words, the handoff should be made with a frequency of an object cell for the existing call but the frequency for the adjacent base station does not exist in the current cell or the mobile station.
A related art method to solve the above-described problem uses a hardware device that transmits all frequencies used in the multiple frequency environment, including the frequencies used in the base station and the adjacent base station. The hardware device is installed and the PSMM is generated when the mobile station is moved to the object cell. In other words, if the PSMM is generated, the base station and the control station analyze the PSMM and process a CDMA system handoff control signal procedure using the hard handoff method.
When the mobile station is moved to the object cell, the mobile station can measure a pilot signal of the object cell and transmit the PSMM as a handoff process determining message. Therefore, the hard handoff between direct frequencies can be achieved in accordance with the CDMA system handoff control signal procedure. In other words, if the PSMM is analyzed and the signal strength of the object cell is fully satisfied with the hard handoff condition, the CDMA system constitutes a new speech path between the mobile station and the object cell and simultaneously retrieves the speech path of the existing cell.
Even if, however, the related art method is widely used, a plurality of the base stations as shown in FIG. 1 exist, for example, in the peripheral area of the multiple frequency environment. The related art method has a problem in that a large amount of installation cost is required for the hardware device.
A second related art method is disclosed in Korean Patent Application No. 96-7664 (Publication Date Jun. 8, 1996) entitled "Handoff determining method of CDMA cellular system". Korean Application No. 96-7664 (hereafter '7664) discloses a handoff process method in which a separate pilot transmission hardware device is not installed in an adjacent base station. That is, the mobile station does not measure the pilot strength of the object cell. The mobile system compares a cell signal during current call with a newly suggested signal strength comparison parameter value. The mobile system transmits the PSMM to achieve the CDMA system handoff control signal procedure if the compared result exceeds a predetermined level.
In the '7664 method, a new threshold value parameter T.sub.-- HRDH, which corresponds to any value between values T-ADD and T.sub.-- DROP, is defined and transmitted to the mobile station. The values T-ADD and T-DROP are handoff pilot threshold values defined and used in a CDMA standard (TIA/EIA IS-95/J-STD-008). At the time, the mobile station does not measure the pilot strength of the object cell and monitors the pilot strength of the existing cell during the call. Here, if the monitored pilot strength value is over the T-HRDH value, the mobile station transmits the PSMM to thereby generate a hard handoff within cells from an existing cell F1 to an object cell F0. Once the call is transmitted to the object cell F0, it generates the soft handoff with the object cell F0.
However, the '7664 method deviates from the CDMA standard between the mobile station and the base station. Accordingly, there is a problem in that it can not be readily applicable because of incompatibility with existing mobile stations.
In a third related art method, Korean Patent Application No. 96-27545 (Publication Date Jul. 22, 1996) entitled "Handoff control method between different cell hierarchies in CDMA cellular system" discloses another handoff process method in which a separate pilot transmission hardware device is not installed in the adjacent base station. That is, the mobile station senses its own frame error rate during call and if the sensed frame error rate is larger than a specific threshold value, processes the handoff from the time point. The mobile station during a call in the existing cell reports the frame error rate and a mean power value to the base station. If the frame error rate is larger than the specific threshold value, processes the hard handoff between the frequencies between the object cell and the existing cell.
However, since the call during a conversation with an arbitrary cell has a good frame error rate in the whole area excepting a boundary area of the terminal base station and the frame error rate is increased without moving to the boundary area if an amount of calls are increased, there is a problem in that if the frame error rate is considered a threshold value to determine the handoff process time point, the handoff time point is too late or is not accurate. The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.